1. Field of the Invention
A microwave dielectric ceramic composition having high dielectric constants, high quality coefficient (Q) values, and small temperature coefficients.
2. Description of the Prior Art
Microwave communication systems such as mobile radio communication, satellite communication and satellite broadcasting systems are currently being rapidly developed. Dielectric ceramics have been extensively utilized in a band-pass filter resonator device or a microwave integrated circuit in these systems, because dielectric resonators contribute to miniaturization, they are light weight, relatively inexpensive, and improve thermal stability.
The most important characteristics required in microwave dielectric materials are:
1. a large dielectric constant (.epsilon.r) for miniaturization, since the size of resonator is inversely proportional to the square root of dielectric constant; PA1 2. a high Q value (=1/tan .delta.), to improve the selectivity of resonant frequency (f.sub.0); and PA1 3. a small temperature coefficient of resonant frequency, to obtain temperature stable resonant characteristics. PA1 1. a composite perovskite type solid solution, such as Ba(Mg.sub.1/3 Ta.sub.2/3)O.sub.3, having high Q values (Q.times.f=200,000) at dielectric constants ranging from 25 to 30. [reference: K. Matsumoto, T. Hiuga, K. Takada and H. Ichimura, "Ba(Mg.sub.1/2 Ta.sub.2/3)O.sub.3 Ceramic with Ultra-low Loss at Microwave Frequencies" In Proc. of the Sixth IEEE International Symposium on Applications of Ferroelectrics, pp. 118.about.121, (1986)]; PA1 2. a simple solid solution, such as (Zn, Sn)TiO.sub.4, having medium Q values (Q.times.f=50,000) and dielectric constants ranging from 30 to 40 [reference: K. Wakino, K. Minai and H. Tamura "Microwave Characteristics of (Zn,Sn)TiO.sub.4 and PbO--Nd.sub.2 O.sub.3 --TiO.sub.2 Dielectric Resonators" J. Am. Ceram. Soc. 67(4), 278-281 (1983)]; and PA1 3. a mixed phase material, such as (Ba,Pb)O--Nd.sub.2 O.sub.3 --TiO.sub.2, having high dielectric constants ranging from about 80 to about 90 but relatively low Q values (Q.times.f=3,000) [reference: K. Wakino, K. Minai and H. Tamura "Microwave Characteristics of (Zn,Sn)TiO.sub.4 and PbO--Nd.sub.2 O.sub.3 --TiO.sub.2 Dielectric Resonators" J. Am. Ceram. Soc. 67(4), 278-281 (1983)].
Known representative microwave dielectric materials are:
Recently, (Pb, Ca)ZrO.sub.2 type ceramics with high dielectric constants, having dielectric constants of 100 to 105 and Q values of about 1,000 (Q.times.f=3,000), have been reported [reference: J. Kato, "Material Produces Small Resonators with High Dielectric Constant", JEE, Sep., 114-118(1991)]
Dielectric ceramics having high dielectric constant are suitable materials for microwave device using electric waves of long wave length and are in great demand for use in devices for cellular phones. However, it is very difficult to develop dielectric materials having temperature coefficients of from negative 30 to positive 30 ppm/.degree.C. which also have high dielectric constants and high Q values. Since materials with high dielectric constants usually have low Q values and large temperature coefficients, it is very difficult to satisfy the three desirable characteristics in a single material system. In fact, a material satisfying the three desirable characteristics simultaneously has not been found.